Numerous methods are known for generating optical pulse trains at a high-repetition rate in which the pulses are nearly transform-limited and have a sech-like shape. Such optical pulses are important, for example, in soliton transmission systems.
For example, M. Nakazawa, S. Suzuki, Y. Kimura, Opt. Lett. 15, 715 (1990) disclose optical pulse sources that employ gain-switched, output-filtered, distributed feedback (DFB) lasers. However, these lasers suffer from extreme chirp and excessive timing jitter. Mode-locked semiconductor lasers using external cavities have also been employed as optical pulse sources and are disclosed, for example, in the following references: G. Eisenstein, R. S. Tucker, U. Koren an S. Korotky, IEEE J. Quantum Electronics 22, 142 (1986); J. E. Bowers, P. A. Morton, A. Mar and S. W. Corzine, IEEE J. Quantum Electronics, 25, 1426 (1989); R. S. Tucker et al., Electron. Lett. 25, 621 (1989); M. C. Wu et al., Appl. Phys. Lett. 57, 759 (1990); and P. B. Hansen et al., IEEE Phot. Technol. Lett. 4, 215 (1992). These sources tend to be difficult to align, have low output power, and are generally expensive to implement in a practical manner.
Other known sources of optical pulses having the abovementioned characteristics include harmonically mode-locked, modulator-driven, fiber ring lasers. These sources, which require servo-control of the ring length, also tend to be expensive and suffer from modulator bias drift. An example of such a source is disclosed in G. T. Harvey and L. F. Mollenauer, Opt. Lett. 18, 107 (1993). Other sources, such as disclosed in M. Suzuki, H. Tanaka, N. Edagawa, K. Utaka, Y. Matsushima, J. Lightwave Technol. 11, 468 (1993), employ an intensity modulator to carve the pulses from a cw source. However, these sources also tend to suffer from modulator bias drift. Finally, some sources employ dual-frequency laser signals which undergo a nonlinear transformation into the desired pulse trains. However, for repetition rates less than 20 GMZ, these sources typically require that the pulse-forming fiber have an impractically long length. An example of such a source is disclosed in P. V. Mamyshev, S. V. Chernikiov and E. M. Dianov, IEEE J. Quantum Electron. QE-27 (2347 (1991).
It is, therefore, an object of this invention to provide a simple and relatively inexpensive method and apparatus for generating high quality pulse trains at a high repetition rate while at the same time avoiding the above-enumerated deficiencies.